Although obesity in general can be predictive of morbidities such as coronary artery disease and diabetes, the anatomical distribution of adipose tissue (fat) can be a strong independent predictor of these and other medical conditions and outcomes. For example, overweight subjects with a larger proportion of fat stored as visceral adipose tissue (VAT) are believed to be at a higher risk than similarly overweight individuals with a larger percentage of fat stored as subcutaneous adipose tissue (SAT). Studies have shown that VAT levels are a predictor of cardiovascular risk factors, e.g. HDL, LDL, triglyceride levels, and hypertension. [1, 2, 3, 4, 10, 11, 12, 13]. (References are identified by numerals in square brackets and are fully cited at the end of this disclosure. The content of the references is hereby incorporated by reference in this patent specification.)
Currently, computerized tomography (CT) and magnetic resonance imaging (MRI) are used to measure or estimate VAT by differentiating it from SAT in abdominal cross-sections or slices. [4, 5, 9, 13] Typically, or at least frequently, this measurement is made at the level of the umbilicus, where SAT and VAT volumes typically are identified by an image thresholding algorithm. However, the relatively high cost of both examinations and the high radiation dosage of CT can discourage the use of these techniques as a screening tool for VAT levels. Further, the thresholding method lacks specificity, because areas or volumes above the threshold can have different amounts of % fat, and areas or volumes below the threshold may not be fat-free. Thus, systematic errors can be introduced by assumptions of % fat in areas or volumes above or below the threshold.
Dual-energy x-ray absorptiometry (DXA) exams are widely available, rapid, relatively low dose, and much less costly than CT and MRI exams. Further, DXA is capable of measuring both global and regional fat mass because, for tissue paths that are projected as pixels in the x-ray image, a given dual-energy x-ray measurement pertains to a unique combination of rat and lean mass. [6, 7, 8, 14, 15, 16] However, because DXA is a two-dimensional projection technique, its ability to distinguish between VAT and SAT has been limited.
Because of the predictive and other values of visceral fat as distinguished from general obesity and subcutaneous fat, and the expense and other disadvantages of known techniques for estimating visceral fat, such as CT and MRI, it is believed desirable to find a way to estimate visceral fat more efficiently and effectively, and this patent specification is directed to meeting that need.